What is distributed control ?
An electrical, electronic or electromechanical system where a
number of devices are “controlled” in some way based on the requirement to
perform a pre-specified set of operations while also , possibly, taking
account of sensor readings, can be described loosely as a control system.
In any control system there is a controller which acts as the “brain”
behind the process. In a typical PLC control system the PLC controller is
a single entity with lots of inputs and outputs suited to a wide variety
of devices and sensors.

Once the PLC has been programmed with the particular requirements of the
application and it has been connected to all its inputs and outputs , it
is ready to control. The PLC would typically repeat the following loop
continuously as it performs its tasks….

1. Read all Inputs
2. Determine New Outputs based on current state and current inputs
3. Set the New Outputs

All processing needed to be done in carrying out this loop would be done
by the PLC’s own local processor. All wiring to and from the sensors and
controlled devices would also be routed to somewhere near the PLC’s
processor. Typically this would be in a cabinet fitted with DIN rails
holding the PLC module and additional modules for connection to the
“devices”.
In contrast to this is the idea of Distributed Control. In this case there
is not just one processor dealing with all of the system. The overall
system is divided up into local processing modules which can read and set
their own attached devices/sensors. Each different module can specialise
in a particular type of device: for example, motor control, digital
input/output, analogue measurement, relay switching etc… etc… However,
each module must have some general guiding control which coordinates the
operation of all of the modules in a given system. This means that there
must be communication between them. This can take many forms but some of
the simplest and most reliable are those using serial multi-drop protocols
such as RS485. This has the convenience of requiring just two wires to
link together all modules in a multi-module system. It should be noted
that the ability to have separate modules linked only be a pair of wires
can greatly reduce the overall wiring effort in a control system since the
modules can be placed near to their own devices.
The “guiding control” can
be provided by a “Master Controller” which has the ability to communicate
with all of the modules in such a system. Although it may seem that taking
this approach to distributed control has just added an extra layer of
complexity to a simple control system, it is the separation of the
“housekeeping “ processing to each of the individual module processors
that makes it very efficient and generally easier to program and maintain.

For example: If the PLC processor needed
to make a measurement of a temperature using one of its attached sensors
it would need to initiate the analogue to digital conversion of the “raw”
input signal before using the result in its main job of deciding what to
do at “such and such” a temperature. When doing the same thing using a
dedicated analogue input module the “Master Controller” can simply request
the current temperature from the module using the communications channel.
The analogue module would carry out the A/D conversion automatically in
the background. Similarly, a stepper motor requires a special sequence of
pulses to each of its phase coils to make it move in a particular
direction at a particular speed. With a separate stepper motor controller
it would simply require the “master” to send a command telling it to take
“so many steps” at “such and such a speed” leaving the modules local
processor to take care of the correct pulse sequence generation.
Another advantage in distributed control is the ability to have the
modules separated by quite large distances without having to worry about
cable lengths and associated power distribution problems. For example: To
operate a large motor requiring large currents at a long distance from the
controller (eg 1Km) would require expensive cable to prevent voltage drops
between the supply and the motor. The local controller option means that
the motor power supply can be situated exactly where it is needed; close
to the motor and controller.